This invention relates to an exposure reflective mask for use in manufacturing a semiconductor device or the like, a reflective mask blank from which the mask is obtained, and a method of manufacturing a semiconductor device by the use of such a reflective mask.
In recent years, as semiconductor devices have been miniaturized more and more, extreme ultraviolet (abbreviated to EUV below) lithography has been considered as an exposure technique that uses EUV light and has been expected to be promising in the semiconductor industry. The EUV light may be defined as light in a wavelength band of the soft X-ray region or the vacuum ultraviolet ray region and, specifically, may be light that has a wavelength of about 0.2 to 100 nm. As a mask for use in the EUV lithography, there has been proposed an exposure reflective mask as described, for example, in Japanese Examined Patent Publication (JP-B) No. H07-27198 (namely, 27198/1995) (will be referred to as Document 1).
Such a reflective mask has a multilayer reflective film formed on a substrate to reflect exposure light and further has an absorber layer formed in a pattern on the multilayer reflective film to absorb the exposure light. The exposure light incident on the reflective mask disposed in an exposure apparatus (pattern transfer apparatus) is absorbed at a portion where the absorber layer is present, while, is reflected by the multilayer reflective film at a portion where the absorber layer is not present. As a result, a reflected optical image from the reflective mask is transferred onto a semiconductor substrate through a reflective optical system.
As the foregoing multilayer reflective film, a stacked film adapted to reflect EUV light of 13 to 14 nm is known which is formed by stacking Mo and Si films each having a thickness of several nm, as shown in FIG. 3. Specifically, the Mo and Si films are alternately laminated in set and stacked by about 40 to 60 sets. In order to increase the reflectance, it is desirable that the Mo film having a high refractive index be formed as an uppermost layer of the multilayer reflective film. However, Mo is quickly oxidized when exposed to the atmosphere and, as a result, the reflectance is lowered. In view of this, as a protective film for preventing oxidation, the Si film, for example, is arranged as the uppermost layer.
On the other hand, Japanese Unexamined Patent Application Publication (JP-A) No. 2002-122981 (will be referred to as Document D2) discloses a reflective mask which has a buffer layer of ruthenium (Ru) formed between an absorbent pattern and a multilayer reflective film composed of Mo films and Si films alternately laminated.
In the case where the Si film is disposed as the protective film at the uppermost portion of the multilayer reflective film as described above, the oxidation preventing effect cannot be sufficiently achieved if the thickness of the Si film is thin. Taking this into account, the Si film is normally deposited to a thickness enough to prevent oxidation. However, this structure brings about a problem that the reflectance is reduced as the thickness of the Si film increases, because the Si film slightly absorbs the EUV light.
Further, when the Ru film is arranged between the multilayer reflective film and the absorbent pattern as described in Document D2, the following problems take place.
(1) As described in Document D2, the Si film is usually arranged as the uppermost layer of the multilayer reflective film and, further, contacts with the Ru film deposited as the buffer layer. With this structure, the Ru film tends to be diffused into the Si film placed at the uppermost layer of the multilayer reflective film and forms a diffusion layer. Such diffusion is caused to occur during deposition of the Ru film and heat treatment or the like thereafter and such a diffused layer undesirably reduces the reflectance.
(2) In the case of the multilayer reflective film in the reflective mask, it is required to have resistance to environment during pattern formation of the absorber layer or environment of pattern formation of a buffer layer when the buffer layer is formed between the multilayer reflective film and the absorber layer. That is, it is also necessary to consider a condition that a material of a protective film formed on the multilayer reflective film should exhibit a large etching selectivity to the absorber layer and/or the buffer layer.
For example, when the absorber layer is made of a Ta-based material, there is a case where a buffer layer of a Cr-based material is formed to avoid etching damage to the multilayer reflective film during pattern formation of the absorber layer and, after the patterning of the absorber layer, the Cr-based buffer layer is also patterned. The Cr-based buffer layer is normally patterned by dry etching by the use of a chlorine-based gas containing oxygen. However, since the Ru film has a low etching resistance to the chlorine-based gas containing oxygen in an amount of 70% or more, the multilayer reflective film is damaged to thereby cause a reduction in reflectance.
(3) It is to be noted here that the protective layer is inevitably and physically reduced in thickness due to etching during the pattern formation of the absorber layer or the Cr-based buffer layer. In recent years, the etching conditions to the protective film tend to be strict in terms of reduction in processing size so that the protective film requires a thickness that can fully withstand the etching for a long time. However, the Ru film has a narrow optimal thickness range in which high reflectance can be achieved and in which the Ru film should have a comparatively thin thinckness. Moreover, even if the Ru film is formed with a thickness within such an optimal thickness range, it cannot withstand etching for a long time so that the multilayer reflective film is subjected to etching damage. As a result, a reduction in reflectance occurs. Further, the thickness reduction of the protective film due to the etching during the pattern formation of the absorber layer or the Cr-based buffer layer is not necessarily constant but varies. Therefore, when the optimal thickness range is narrow like the Ru film, it is quite difficult to set an initial thickness of the Ru protective film so that the Ru protective film fully withstands the long-time etching and further the thickness of the Ru protective film after the etching falls within the optimal thickness range. Consequently, the reflectance tends to be lowered due to the thickness of the Ru protective film after the etching.